1. Nuclear Chemistry

2. What’s in an atom? What’s its mass? What’s its charge?
Where it’s found
Particle
Proton
In nucleus +1
~ 1 amu
Neutron
In nucleus
~ 1 amu
Electron
Flying around nucleus -1
~ 1/1840 amu

3. Basic Terminology Recall: Z = atomic number = # of protons
N = number of neutrons
A = mass number = Z + N
Since each proton and neutron weighs roughly 1 amu, the mass of a nucleus (in amu) is roughly equal to its mass number.

4. In the Nucleus Most of the mass of the atom VERY small volume
radius ~ m
Number of protons determines identity
Number of neutrons in an element can vary
Isotopes: atoms with = protons but different numbers of neutrons
Average atomic mass = weighted average of the masses of all naturally-occurring isotopes.

5. In the Nucleus Electrostatic forces: protons repel each other
“Strong” nuclear forces: attract protons to neutrons
The balance between these two forces determines whether or not a given nucleus will be stable.
Strong force explanation
More on strong and weak forces

6. Unstable Nuclei wrong Z/N balance,  radioactive decay
i.e. the atom will emit radiation.
Radioactive nuclei spontaneously decay .
The degree of unbalance determines the type of decay
Radioactive nuclei (parent nuclei) decay into daughter nuclei.

7. Types of Radiation Alpha Radiation Beta Radiation Gamma Radiation
a positive charged particle is released
a helium nucleus
Beta Radiation
a negatively charged particle is released
an electron
Gamma Radiation
no particle is released
high energy waves are released

8. Alpha ()Decay
The nucleus is too large for the strong force to hold it together,
an a-particle (24He nucleus) is emitted
Daughter nucleus has 2 fewer protons and 2 fewer neutrons than parent.
Alpha radiation is too weak to penetrate paper or skin
Nuclear equation: U  24He Th

9. Beta ()Decay When a nucleus has too many neutrons,
a b-particle ( -10e) is emitted, a neutron in the nucleus splits into a p+ and an e-
The p+ stays in the nucleus.
The e- is ejected and called a b-particle.
Daughter nucleus has 1 more proton and 1 fewer neutron than parent.
Nuclear equation:
614C --> 714N + -10e
Beta radiation is unable
to penetrate aluminum foil
or wood

10. Gamma () Radiation
When a nucleus has too much energy, it can give off very high energy waves of light.
the nucleus is unchanged - it still has the same # of p+ and # of n0.
Nucleus goes from “excited state” to “ground state,” losing excess energy.
Gamma rays are often given off
with other types of radioactivity.
Gamma radiation can pass
through people

11. Radiation Shielding

12. Electron Capture When a nucleus has too few neutrons,
An electron falls into the nucleus
unites with a proton to form a neutron.
Electrons cascade in to fill in for the missing electron
Daughter nucleus has 1 more neutron and 1 fewer proton than parent.
Nuclear equation:
69C + -10e --> 59B

13. Chapter 18 Gamma Rays Are Also Emitted in Positron Emission
Section 2 Nuclear Change
Gamma Rays Are Also Emitted in Positron Emission
Some nuclei that have too many protons can become stable by emitting positrons.
Positrons are the antiparticles of electrons.
The process is similar to electron capture in that a proton is changed into a neutron.
However, in positron emission, a proton emits a positron.

14. When a proton changes into a neutron by emitting a positron, the mass number stays the same, but the atomic number decreases by one.
The positron is the opposite of an electron.
Unlike a beta particle, a positron seldom makes it into the surroundings.
Instead, the positron usually collides with an electron, its antiparticle.

15. Any time a particle collides with its antiparticle, all of the masses of the two particles are converted entirely into electromagnetic energy or gamma rays.
This process is called annihilation of matter.
The gamma rays from electron-positron annihilation have a characteristic wavelength.
Therefore, these rays can be used to identify nuclei that decay by positron emission.

16. Radioactive Decay, continued
Chapter 18
Section 2 Nuclear Change
Radioactive Decay, continued
Stabilizing Nuclei by Converting Neutrons into Protons

17. Same nucleus, just lower energy
Radiation - Summary
Problem
Decay Type
Symbol
Resulting Daughter
Nucleus is too large
alpha
a or 24He
Atomic number -2
Mass number -4
Too many n0 in nucleus
beta
b- or -10e
Atomic number +1
Mass number stays same
Too much energy
gamma emission g
Same nucleus, just lower energy

18. Radiation - Summary Problem Decay Type Symbol Resulting Daughter
Too many Protons
Positron
emission
b+ or +10e
Atomic number -1
Mass number stays same

19. Stable Nuclei For small isotopes N ≈ Z
e.g.: 16O is most stable isotope of O:
Z = N = 8 or N/Z=1
For larger isotopes, N/Z is between 1 and 1.5
e.g.: 208Pb is most stable isotope of Pb:
Z = 82; N = 126 or
N/Z=1.5

20. Practice
Nuclear Equations

21. Half-Life amount of time it takes for half of a given sample to decay.
Each half-life, half of the sample decays and half remains.
Half lives vary from billionths of a second to billions of years.

22. Half-Life: Equation Form
How do we put this into equation form?
Let t1/2 = the half-life of the isotope
Let A0 = the amount you start with
Let A(t) = the amount remaining at time t
Then t/t1/2 = the number of half-lives that have elapsed.
A(t) = A0 (1/2)t/t1/2

23. Nuclear Reactions - Fission
If nucleus too big - nuclear fission.
“fissions” (breaks up) into several smaller nuclei (and usually some extra neutrons as well).
not easily predicted.
Often initiated by absorbing a neutron.
Example: 92235U + 01n --> 56141Ba Kr n

24. Nuclear Energy - Fission
Energy can be harnessed.
92235U + 01n --> 56141Ba Kr n
The neutrons collide with other atoms of 235U,
split, producing more neutrons . . .
causing a chain reaction.
The energy given off can be harnessed to produce electricity (or, unfortunately, for more destructive purposes).

27. Nuclear Energy - Fission

28. Nuclear Weapons

29. Nuclear Waste
This pool at the Areva Nuclear Plant near Cherbourg, France, cools spent nuclear fuel rods before they are moved underground.
Francois Mori / AP

30. Nuclear Fusion
The opposite of nuclear fission is fusion, when smaller nuclei come together to form larger nuclei.
Example: 11H + 13H --> 24He
The fusion of hydrogen to form helium is the source of energy for the sun and many other stars.

31. Nuclear Energy - Fusion
Release even more energy than fission.
emitted by stars (mostly hydrogen fusing to form helium).
no safe way yet to harness fusion
takes too much energy to get started

32. Energy in Radioactive Decay
Radioactive Decay generally gives off large amounts of energy.
Where does it come from?
The answer lies in something called “mass defect.”

33. Mass Defect Law of Conservation of Mass Law of Conservation of Energy
Mass cannot be created or destroyed.
Law of Conservation of Energy
Energy cannot be created or destroyed.
But, during nuclear reactions:
Mass can be converted into energy and energy can be converted into mass.

34. where c = the speed of light = 3.0 x 108 m/s.
Mass Defect
During nuclear reaction, some mass is either lost or gained. This change in mass is called the mass defect (Dm).
Dm = mass of products - mass of reactants
The relationship between the mass defect and the amount of energy given off or absorbed (DE) is
DE = Dmc2
where c = the speed of light = 3.0 x 108 m/s.

35. Mass Defect
DE = Dmc2
Example: If 0.01g of mass is converted into energy, 9 x 1011 J of energy is given off. That’s enough to heat 2.7 million liters of water from room temperature to boiling!
By comparison: you would need to burn 16 million grams of methane to give off the same amount of heat.

36. Ten most common elementsZ
Element
% Composition 1
Hydrogen
74% 2
Helium
24% 8
Oxygen 1% 6
Carbon
0.4% 10
Neon
0.1% 26
Iron 7
Nitrogen
0.09% 14
Silicon
0.06% 12
Magnesium
0.05% 16
Sulfur
0.04%
Ten most common elements
in the Milky Way

38. Stellar Nucleosynthesis – the Stellar Metals
H, He, Li
H is exhausted,
He is burned, fission creates
Li, B, Be, C
carbon burning white dwarfs
more massive stars use nuclear fusion to create
N, O, F, Ar, Na and Mg
most massive stars use nuclear fusion to create
Al, Si, P, S, Cl, Ar, K, Ca, Sc, Ti, V, Cr, Mn, Fe

39. Stellar Nucleosynthesis – beyond Z=26 . . .
Most massive stars (12x our sun)
burn through Mg, Si etc,
only Fe core remains
implode from its own gravity
matter is crushed
p’s and e’s form n’s
explode during which
Fe nuclei gain n’s then decay into protons
new elements (thru 92) are formed

40. Common Uses of Radioactivity
Food Irradiation
Archaeological Dating
Medical Detection
Medical Treatment

41. Daily Exposure to Radiation
Cosmic rays
Radon gas
Smoke detectors

42. Detecting Radiation Geiger Counters Scintillation Counters Film (beta)
(alpha, beta and gamma)
Film
(beta, gamma)